When an embryo is dividing, how do cells know what function they are going to
perform?

Murray Grigor, of the Centre for Gene Research at Otago University, responded.

The fertilised egg is, of course, a single cell that contains in its DNA
all the information needed for the development of the mature animal.

Early in embryonic development the cells begin to show differences which
indicate that they are destined to develop into specific organs and
tissues. The differences amongst the cells in these tissues result from
the fact that only a certain set of genes are being expressed, that is
are active, in them. The term differentiation is used to describe this
process. The control of this has puzzled scientists for a long time and
now we are starting to understand what is going on.

Firstly the fertilised egg is not symmetrical and this asymmetry can signal to the
cells their first decisions as to their fate. Subsequently, cells can
"talk" to one another by producing molecules that act as signals to
modify gene expression in target cells. Sometimes they send signals to
themselves that ensure they continue to develop along appropriate lines.
Studies with the fruit fly Drosophila have provided much of the
information about which genes are activated at particular times within
the development period. These genes contain regions that can bind
specific regions of the DNA controlling the expression of other genes.
They are often named after developmental abnormalities detected resulting
from mutations in specific genes. For example, one gene is called
antennapedia, because it can lead to the formation of a leg (pedia) where
an antenna should normally be. Although humans don't have antennas, we
do have similar genes that are believed to play a different but critical
role in human embryonic development.